This invention relates to a functional molecular element that changes in conductivity upon application of an electric field, a process for its fabrication, and a functional molecular device.
Nanotechnology is to observe, produce and/or utilize a microstructure the size of which is of the order of one one-billionth of a meter (10−8 m=10 nm).
An extremely high-accuracy microscope called “scanning tunneling microscope” was invented in the late 1980's, and made it possible to observe atoms and molecules individually. The use of a scanning tunneling microscope has made it possible not only to observe atoms and molecules but also to manipulate them individually.
For example, writing letters with atoms arranged on a crystal surface has been reported. Despite its capability of manipulation of atoms and molecules, it is not practical to create a new material or to assemble a new device by manipulating an immense number of atoms and molecules individually.
Making a structure of nanometer size by manipulating atoms or molecules or groups thereof individually requires a new technology for ultraprecision fabrication. Roughly categorizing such microprocessing technologies with accuracy of the order of nanometers, two methods are known.
One is the so-called top-down method which has conventionally been used to fabricate various semiconductor devices. It is exemplified by fabrication of integrated circuits from a large silicon wafer by extremely accurate etching to the very limits. The other is the so-called bottom-up method, which is designed to assemble a desired nanostructure from atoms and molecules as extremely small constituents.
The limit of size of nanostructure that could be achieved by the top-down method is suggested by well-known Moore's law (proposed in 1965 by Gordon Moore as a co-founder of Intel Corporation) which stipulates that “the number of transistors on a chip will be doubling every 18 months.” Over the past 30 years since 1965, the semiconductor industry has increased the rate of integration of transistors as predicted by Moore's law.
The International Technology Roadmap for Semiconductor (ITRS) for Coming 15 Years, which was published by the Semiconductor Industry Association (SIA), includes a view that Moore's law will continue to remain effective.
As the degree of microfabrication increases further, the resulting semiconductor chips run faster with less power consumption. Moreover, improved microfabrication yields more products from a single wafer, thereby making it possible to reduce the production cost. This is the reason why microprocessor makers compete in the process rule for new products and the degree of transistor integration.
It is, however, indicated that Moore's law will reach a limit before too long in the light of natural rule.
According to the current major semiconductor technology, for example, semiconductor chips are fabricated by forming circuit patterns on silicon wafers in accordance with a lithographic technology. It is, therefore, necessary to improve the resolution for further miniaturization. To this end, it is necessary to develop a practical technology to employ light with a shorter wavelength.
Another problem involved in increasing the degree of integration is excessive heat evolution per semiconductor chip, which leads to malfunction or thermal breakage of heated semiconductor chips.
Moreover, there are some experts who predict that miniaturization of semiconductor chips that continues at the present pace will reach the stage in which equipment cost and process cost go up and yields go down. As a result, the semiconductor industry will not pay in around the year 2015.
A still more serious problem which has been pointed out recently is line edge roughness (or minute irregularities around pattern edges). As to irregularities on the surface of a resist mask, it is said that the size of molecules constituting a resist and the distance of diffusion of an acid in a chemically amplified photoresist become critical as the pattern size is reduced more than before. The magnitude of a cycle of irregularities on a pattern edge has also been evaluated for its effect on the characteristics of a device, and has arisen as another important problem.
As a new technology to resolve the above-mentioned bottleneck of the top-down method, attention is attracted to research that attempts to provide each molecule with a function as an electronic part. Illustrative is an electronic device (molecular switch or the like) fabricated from individual molecules by the bottom-up method.
Concerning metals, ceramics, or semiconductors, research is also under way to make structures of nanometer size by the bottom-up method. It would be possible to design and create, by the bottom-up method, (molecular) devices entirely different in characteristics from ones in the related conventional art if good use is made of millions of species of diverse molecules independently varying in size and function.
For example, conductive molecules have a width as small as 0.5 nm. Lines of these molecules can realize thousands times high-density wirings compared with the line width of 100 nm or so achieved in the current IC technology. Moreover, it would be possible, for example, to realize a recoding device with a capacity larger than 10,000 times that of DVD if individual molecules are used as memory elements.
Molecular devices are synthesized by a chemical process unlike conventional silicon semiconductors. The world's first organic transistor of polythiophene (polymer) was developed in 1986 by Hiroshi Koezuka of Mitsubishi Electric Corporation.
Further, a group of researchers from Hewlett-Packard Company and the University of California at Los Angeles in the US succeeded in developing organic electronic devices, published them in the July 1999 issue of Science, and filed patent applications on them (see Patent Document 1 and Patent Document 2 to be described subsequently herein). They also made switches from molecular films composed of millions of rotaxane molecules (which are organic molecules) and completed an AND gate, a fundamental logic circuit, by connecting them together.
A joint research group between Rice University and Yale University in the US succeeded in creating a molecular switch the molecular structure of which changes upon injection of electrons under the effect of an electric field to perform a switching operation. They published it in the November 1999 issue of Science (see Non-patent Document 1 to be described subsequently herein). It has a function to repeatedly perform on-off switching, which was not achieved by the group of researchers from Hewlett-Packard Company and the University of California at Los Angeles. In addition, it has one millionth of the size of ordinary transistor, and this smallness will contribute to the manufacture of small high-performance computers.
Prof. J. Tour (of Rice University, chemistry) who succeeded in the synthesis suggests that the fabrication cost of molecular switches would be only one thousandth of that of conventional semiconductors because of the obviation of expensive clean rooms which would generally be used for the fabrication of semiconductors. He is planning on constructing a hybrid computer (composed of organic molecules and silicon) in five to ten years.
A great deal of research has been carried out on molecular devices with functions of electronic components as mentioned above. However, most of the past research on molecular devices is directed to those which are driven by light, heat, protons, ions or the like (see, for example, Non-Patent Document 2 to be described subsequently herein), and those which are driven by an electric field are limited in number.
With respect to these molecular devices, the above-mentioned problem of line edge roughness also arises as a serious problem, and is expected to become more serious as the pattern is miniaturized further. As a method for avoiding this problem with respect to molecular devices, it is a common practice to introduce a thiol group into a terminal of a molecule for direct coupling to a gold electrode (see, for example, Non-Patent Document 3 to be described subsequently herein). Molecular devices are superior in reproducibility to those of inorganic materials because their molecules themselves are smaller in size than the minimum unit that causes the problem of roughness.
However, the electrical connection between the thiol group and the gold electrode involves a problem in that, no matter how good electrical properties the molecule itself has, the connecting part between the terminal thiol group and the electrode has large electric resistance and this large electric resistance limits improvements in the characteristic properties of the molecular device (see Non-Patent Document 4 to be described subsequently herein).
In the development of molecular devices, it has been studied to use a variety of organic molecules. Illustrative are a group of compounds called “tetrapyrroles” each of which contains four pyrrole rings. Tetrapyrroles include those having cyclic structures and those having acyclic structures. Those having cyclic structures include, for example, porphyrin, which has a ring (tetrapyrrole ring) that four pyrrole rings are linked together via one carbon atom between each two adjacent ones of the pyrrole rings, and its derivatives. It is known that porphyrin and its derivatives form stable metal complexes with many metal atoms and each of these metal complexes takes a stacked structure with planes of porphyrin rings being stacked one over the other. Those having acyclic structures, on the other hand, are called “acyclic tetrapyrrole,” “ring-opened tetrapyrrole” or “linear pyrrole” in which four pyrrole rings are linearly linked together via one carbon atom between each two adjacent ones of the pyrrole rings.
A description will hereinafter be made about certain examples of reports on tetrapyrrole-containing molecular devices.
Referring first to Patent Document 3 entitled “Photo-Functional Molecular Element with Porphyrin Polymer Fixed and Stacked on Substrate through Covalent Bonds and Method of Preparing the Same” and to be described subsequently herein, it is described that subsequent to application of a linker polyphyrin onto a gold substrate, the substrate is immersed in a solution of an imidazole-substituted zinc porphyrin to stack a polyphyrin polymer through coordinate bonds.
Patent Document 4 entitled “Functional Molecular Element” and to be described subsequently herein contains descriptions as will be described hereinafter.
There is a description about a functional molecular element constructed by using a system which changes in the anisotropy of dielectric constant by a change in molecular structure as induced by an electric field. It is described to be preferable for such a functional molecular element to use molecules of an organic metal complex formed of an organic molecule, which is equipped with dielectric constant anisotropy and changes in structure under the effect of an electric field, and a metal ion. The organic molecule may have, for example, linear side chains, and may desirably be discoid (or nearly discoid).
The functional molecular element is described to preferably form a columnar array structure that nearly discoid, organic metal complex molecules having side chains are arrayed in a columnar form between a pair of mutually-opposing electrodes. It is also described that the nearly discoid, organic molecule having the side chains may preferably be that of a biladienone derivative such as biliverdine or biladienone and the metal ion may preferably be a zinc ion, copper ion, nickel ion or the like.
Further, it is described that a biline derivative, phlorin derivative, chlorin derivative or the like is also usable besides the biladienone derivative and that as a metal, another representative element or transition metal is also usable.
Patent Document 5 entitled “Linear Tetrapyrrole Dye” and to be described subsequently herein contains a description about a near tetrapyrrole dye primarily characterized in that it is synthesized by oxidizing and cleaving a tetraphenylporphyrin compound containing an alkyl or alkoxy group on each phenyl group.
In addition, Patent Document 6 entitled “Functional Molecular Element, Method for Producing Functional Molecular Element, and Functional Molecular Device” and to be described subsequently herein contains descriptions as will be described hereinafter.
The invention of Patent Document 6 relates to a functional molecular element in which π-electron conjugated molecules, each of which has a skeleton having a planar or substantially planar structure of a π-electron conjugated system and has side chains bonded to the skeleton, are adsorbed at the side chains thereof on electrodes to form adsorbed molecules arranged such that the planar or substantially planar structures of the skeletons lie substantially in parallel to the electrodes, and a structure formed of at least the adsorbed molecules and the electrodes has a function to pass a current in a direction intersecting the planar or substantially planar structure; and also to its fabrication process, that is, a process for fabricating the functional molecular element, which includes a step of preparing a solution of the π-electron conjugated molecules at an adjusted concentration, a step of bringing the solution into contact with the electrodes, and a step of evaporating the solvent from the solution such that the π-electron conjugated molecules are formed in layers stacked between surfaces of the electrodes as many as corresponding to the concentration.
The invention of Patent Document 6 also relates to a functional molecular device in which the structure constituting the functional molecular element has mutually-opposing electrodes as the electrodes.